Such an apparatus has conventionally been known for evaluating the pulse and blood pressure by calculations made after transmitting light through the tissue of a finger tip, detecting a change in the transmittance (reflectance) of light in the blood in the finger and then processing a detection signal thereof. One example of a photosensor employed in this apparatus is disclosed in Japanese Utility Model Laying-Open No. 60-158803.
FIG. 11 is a perspective view of the photosensor disclosed in the above Japanese Utility Model Publication; FIG. 12 is a cross sectional view of the photosensor shown in FIG. 11; FIG. 13 is a cross sectional view showing the photosensor of FIG. 11 attached to a finger; and FIG. 14 is a perspective view of a fixing tape for securing the photosensor to a finger.
First, a conventional photosensor 8 will now be described with reference to FIGS. 11-14. The photosensor 8 shown in FIG. 11 transmits light from one side to the other side of the finger and detect a change in the magnitude of this transmitted light depending on the blood content in the finger. A light emitting element 2 and a light receiving element 3 are spaced a predetermined distance, corresponding to the size of the finger, apart from each other on a flexible film substrate 1. A transparent flexible light transmissive film 6 is attached to the film substrate 1 so as to cover the light emitting and receiving elements 2 and 3.
The usage of the photosensor 8 shown in FIG. 11 will be described with reference to FIGS. 12-14. The photosensor 8 is wound around a finger 7 so that the tip of the finger 7 is sandwiched between the light emitting element 2 and the light receiving element 3, as shown in FIG. 12. The fixing tape 9 shown in FIG. 14 is then wound on the photosensor 8 and around the finger. This fixing tape 9 is rectangular and comprises physical adhesion means on a front surface of its one end and on a reverse surface of the other end. Winding this tape 9 on the photosensor 8 to lap the front surface of one end over the reverse surface of the other end causes the photosensor 8 to be firmly secured to the finger 7. When a power is supplied to conductors 4 through a connector 5 of a signal processing apparatus (not shown), the light emitting element 2 emits light. This emitted light is transmitted through the finger 7 and directed to the light receiving element 3. The light receiving element 3 receives this light and supplies a detection signal to the main body of the signal processing apparatus through the lead 4 and the connector 5. The signal processing apparatus detects a change in the transmittance at this time, processing the detected signal and then performing a calculation to provide a pulse and a blood pressure value.
The light emitting element 2 employed in the above photosensor 8 generally uses a light emitting diode. The light emitting diode is liable to have its output power and the wavelength of emitted light change depending on ambient temperature. In addition, attaching the photosensor 8 onto a living body, e.g. the finger 7 renders the living body ischemic or hemostatic, so that the body temperature of the finger 7 is decreased, or increased due to an increase in blood pressure. Thus, the ambient temperature of the light emitting element 2 changes, and consequently the output power or the luminescence wavelength change. However, the output power or the luminescence wavelength of the light emitting element 2 is required to be kept constant in order to correctly measure a pulse value, a blood pressure value and the like.